In satellite positioning system, a position of a receiver is determined by passively measuring positioning signals transmitted from a plurality of satellites by the receiver. In this case, time synchronization is one of key technical elements. An onboard clock is used for generation of a regular, generally sequential, series of events, called “epoch”, and a clock time of occurrence of the epoch is coded to a random number code or a pseudo-random number code (hereinafter, referred to as a spreading code). Then, as a result of the pseudo-random number or random number function of a time-epoch coding sequence, an output signal spectrum is determined according to a rate of change of a spread code element and a waveform of a spreading signal. The frequency covers a wide range. Generally, the spreading waveform is of a rectangle (rectangular) and has a power spectrum represented by a sinc function.
An example of such a satellite positioning system includes a global positioning system (GPS). The GPS generally operates by using a plurality of frequency bands such as L1, L2 and L5, each having a center frequency at 1575.42 MHz, 1227.6 MHz and 1176.45 MHz, respectively. Each signal in these frequency bands is modulated by a respective spreading signal. As can be readily understood by those having ordinary skill in the art, a CA (Coarse Acquisition) signal emitted by a GPS satellite navigation system is broadcasted in the 1575.42 MHz L1 band, and has a spreading code rate (chip rate) of 1.023 MHz.
In contrast, apart from the satellite positioning systems including the GPS, there has been known an Indoor Messaging System (IMES) intended to determine position information in an indoor environment. The IMES signal, which is similar to a GPS positioning signal, is broadcasted with the same L1 frequency band of 1,575.42 MHz and has a spreading code rate (chip rate) of 1.023 MHz which is of the same family (Gold series) as a spreading code of the CA signal.
IMES transmitters for transmitting the IMES signal, which are installed in large number in buildings and underground shopping areas, transmit their position information with the IMES signals superimposing it thereon. That is, a user having an IMES receiver is capable of knowing his/her own position by receiving and demodulating the IMES signal and thereby translating the position information superimposed thereon.
In this regard, a CA code of the IMES signal is the same as a CA code of the GPS signal and repeats a series of 1,023 bits (1023 chips) on a cycle of 1 ms. Consequently, in order to switch signals without searching for a carrier frequency and a code phase, it is required for the carrier frequency to have a difference from an expected value falling within a breadth of 1 kHz that is a reciprocal number of the code cycle of lms and, therefore, to be certainly capable of precision within ±500 Hz. Since this represents that the clock frequency deviation is 500 Hz/1575.42 MHz=0.33E-6, it is possible to consider that the degree of precision is required to be less than 2E-6(0.2 ppm) with some margin. Further, since the code phase measures about one microsecond for one chip span, the degree of precision is required to be around ±300 ns or less.
FIG. 8 illustrates a situation in which a user having a conventional IMES receiver moves from a signal area for a conventional transmitter A to a signal area for a transmitter B. When a user having the IMES receiver 803 moves from a signal area (801E) of the transmitter A (801) to a signal area (802E) of the transmitter B (802), it is necessary for the IMES receiver 803 to switch a receivable signal from a signal a compliant with the transmitter 801 to a signal b compliant with the transmitter 802. In this way, in the event of switching the receivable signal from a signal a to a signal b by way of example, it is desired from the viewpoint of communication stability and user convenience that a duration of disconnection from reception of IMES signals is as short as possible.
In consequence, for making the duration of signal disconnection as short as possible, it is required for signals a and b sent from the IMES transmitters A (801) and B (802) to have small differences in carrier frequencies and spreading code phases.
At this point, in order to be capable of receiving IMES signals, the receiver internally generates a signal called a replica signal consisting of frequency and spreading code identical with those of the signal which the IMES transmitter sends out and then performs demodulation while preserving a correlation with broadcast signals. A typical positioning signal receiver is illustrated in block configuration in FIG. 9. The positioning signal receiver 900 in FIG. 9 includes an antenna 901 for receiving signals, a receiving section 902 for performing amplification processing of the received signals through the antenna 901, a reception processing including down-conversion and A/D conversion and others and conversion to a digital intermediate frequency signal (digital IF signal; IF: Intermediate Frequency), a code replica generator 904 for generating code replica signals, and multipliers 905 and 906 each of which multiplies a signal from the receiving section 902 and a signal from the code replica generator 904.
Further, the positioning signal receiver 900 includes a carrier wave replica generator 907 for generating a carrier wave replica signal within the receiver, multipliers 908 and 909 which multiply outputs from the multipliers 905 and 906 by a sin ωrt signal and a cos ωrt signal, i.e. carrier wave replica signals, different 90 degrees in phase from each other from the code replica generator 904, respectively, and further includes an integrator 910 for integrating outputs from the multiplier 908 for a predetermined period of time, an integrator 911 for integrating outputs from the multiplier 909 for a predetermined period of time, and an operation controller 912 for performing integration of outputs from the integrators 910 and 911 (integration before squaring and integration after squared) for S/N ratio improvement in a softwarewise (or computer-programmatic) approach and further controlling the code replica generator 904 and the carrier wave replica generator 907 for signal complement and signal tracking.
At this point, the operation controller 912 is capable of modifying a code generated by the code replica generator 904 in a softwarewise (or computer-programmatic) approach. Further, the operation controller 912 extracts navigation messages based on a received satellite positioning signal and performs positioning operation et. al.
In the process of demodulation, this receiver performs a frequency search for finding frequencies of the broadcast carrier wave and the replica signal carrier wave which come to be coincided with each other (more properly, at a precision within ±500 Hz as described before) and a code phase search for finding code phases of the spreading code sent out from the INES transmitter and the spreading code of the replica signal which become to be coincided with each other. As illustrated in FIG. 10, when the replica signal and the broadcast signal come to be coincided with each other in carrier frequency and code phase, the correlation with the broadcast signal is maximized in value and then the broadcast signal can be received.
In addition, for signal switching without performing these carrier frequency search and code phase search, it was as just described above that the precision of carrier frequency is needed to be about 0.2E-6(0.2 ppm) or less and the precision of code phase is needed to be around ±300 ns or less.